Manufacture of stainless iron



Patented Jan. 21, 1930 WALTER M. IARNSWORTH, QF CANTON, OHIO, ASSIGNDRTO CENTRAL ALLOY STEEL CORPORATION, OF MAFSSILLON, 01-110, A GORPORATIONOF NEW YORK MANUFACTURE OF STAINLESS IRON No Drawing. Application filedJune 29,

This invention relates to the manufacture of those low-carbon,high-chromiumsteels, characterized by resistance to corrosion, tarnishand heat, and which, because'of these properties and their low carboncontent, are variously designated as stainless, stablesurface orrustless irons.

'lhe usual'commercial limits for stainless iron are 0.07 to 0.12% carbonand 12.00 to 18.00% chromium, and one of the chief diffic ulties withwhich manufacturers of this product are confronted is the difficulty ofholding the carbon to these low figures. Because of this difficulty ithas been practically 1927. Serial No. 202,469.

the chromium, only about half of which oxidizes and passes into theslag, the other half remaining as an alloying element in the bath. Atthe conclusion of this first or oxidation stage of the process thecarbon content of the bath is at or below the permissible maximum whilethe chromium content is much below the permissible minimum. Then followsthe second or reducing stage of the process during which the chromiumoxide in the slag is reduced, and the chromium, except for a small less,returned to the bath.

lln carrying out the process, an electric arc furnace of the Heroulttype, or its equivalent,

impossible to make stainless iron by remeltis preferred. I havesuccessfully used a 2-- mg stainless iron scrap, for unless extremeprecautions are taken the scrap, during the re melt, invariably picks upcarbon from the electrodes, the furnace atmosphere andelsewhere,-sometimes up to 0.10%, And it has been impossible to rid thebath of this carbon without, at the same time, experiencing an excessivechromium loss. llt has also been practically impossible to makestainless iron by adding the relatively inexpensive high carbonferro-chrome (containing say 4: to 6% carbon) to a bath of moltenlow-carbon steel because in oxidizingthe excess carbon to bring thecarbon content of the bath down to the permissible maximum the chromiumis also oxidized. Consequently it is common practice to use the muchmore expensive low carbon ferro-chrome containing not over 0.10% carbon.I

his the object of the present invention to make stainless iron withinthe prescribed low carbon limits without experiencing excessive chromiumlosses, and in particular to make stainless iron by remelting stainlessiron scrap.

In attaining this object I proceed as follows: The bath containingcarbon and chro miuin is superheated and subjected to strongly oxidizingconditions. As the temperaturerises the afiinity of carbon for oxygenincreases faster than that of chromium so that under the intenselyoxidizing conditions obtaining in the bath at this stage of the processa very large part of the carbon is completely eliminated without-acomplete oxidation of ton Heroult furnace of standard design equippedwith a Westinghouse automatic regulator and operated with three 8-inchcarbon electrodes on 3-phase current at 110 volts. Abasic refractorylining such as is ordinarily used in furnaces for the manufacture ofalloy steels, may be used, but a bottom put in by burning in successivelayers of ma'gnesite or dolomite will not stand up under continuousoperation at the high temperatures necessary to maintain the necessaryhighly oxidizing conditions in the metal bath. A bottom built up ofchrome brick fused in with a layer of finely crushed chrome ore standsup well during the oxidizing period, but when the reducing materials areadded, the fluid slag which is formed, cuts into and damages the banksand causes considerable difficulty in obtaining uniform results in thechromium content of the bath. 1 have found that these difficulties areentirely overcome by ramming in a bottom consisting of granular basicrefractory material, averaging 94 to 95% MgO, mixed with a binder ofsodium silicate, and fusing it into place. The roof is constructed ofsilica brick in accordance with regular practice.

The scrap is preferably charged into the furnace with a part of theoxidizing material, which material should preferably be the purest formof iron oxide commercially available, such as clean roll scale. Specialiron ore of low silica content may be used but when iron ore of theordinary grade is used, con siderable difficulty is encountered in properly balancing the reducing materials, owing to the variables presenttherein. During the melting down period and at the ordinary refining.temperature, there is no oxidation of carbon, the chromium beingoxidized more readily thanthe carbon, but when the temperature isincreased, the speed of reaction between the oxygen and carbon increasesmore rapidly than the speed of reaction between the oxygen and chromium,thus making oxidation of the carbon possible. The ordinary refiningtemperature is about 1500 to 1600 C. The temperature necessary to effecta reaction between the oxygen and the carbon in the presence of thechromium is usually from to 250 higher. At the time the temperatureis-raised the remainder of the iron oxide is added, the total chargeamounting to about 20% of the weight of the scrap about 450 pounds perton (2240 pounds). This higher temperature must be maintained until thecarbon is reduced to the desired point, and when this is accomplished,the current input to the furnace is reduced as low as possible and thetemperature permitted to drop. Meanwhile a substantial part of thechromium has been oxidized and forms with the iron oxide a heavy layerof slag usually carrying from to Cr O I now come to the second step ofthe process. This stage of operation is begun I with a highly heatedmetal bath above which is a slag containing chromium oxide and ironoxide as essential constituents. The metal bath should contain 6 to11.5% chromium and not more than 0.12% carbon. The chromium oxidecontent of the slag is derived by oxidation during the first step ofoperation, from the chromium originally contained in the scrap. Theaddition agents used during the second step of operation are burnt lime(CaO) and pulverized ferro-silicon. The exact manner in which theseadditions are made may be varied at will to a considerable degree.However, I prefer to make an initial addition of lime which is spreadover the surface of the bath. A mixture of lime and pulverizedferro-silicon is then fed gradually into the bath, preferabl around orbeneath the electrodes. The pu verized ferro-"s'ilicon reduces thechromium and iron oxide contained in the slag and forms silica, whichacidic oxide is in turn neutralized, combining with the lime present. Asa result, the slag which at the beginning of this operation was black ordark brown incolor due to its high percentage of iron and chromiumoxides gradually verges towards a basic character and acquires a lightercolor. High recoveries of chromium may be obtained without actuallyconverting the original black slag into a greenish or grayishwhite slagof the disintegrating type. The ratio of lime to ferrosilicon (50%)which leads to the best results lies between 1.5 and 3.0. At thecompletion of the second step, or reducing period, of operation the slagshould not contain more than 2% oxide of chromium and 2% oxide of iron.

If the reduction process is not efiiciently con-' ducted, however, thesepercentages will be higher. The amount of residual chromium oxide in theslag corresponds to the net loss of chromium for the operation as awhole. The silica (SiO content of the slag at the end of the reducingperiod lies as a rule between 25 and 35%.

Alloying ingredients, necessary to bring the metal up to the desiredspecifications, are then added to the furnace, or if desired in theladle prior to tapping. If the bath is too hot additional stainless ironscrap may be added to cool it. I

I shall now give a specific example of an actual heat. 3000 pounds ofsteel scrap having an approximate analysis of .08% carbon; 35%manganese;.020% sulphur;.020% phosphorus; 16.5% chromium and .80%silicon, were charged into a two-ton furnace with 300 pounds of millroll scale. As soon as the charge was melted, a sample was taken foranalysis, which ran .17% carbon, and 14% chromiuma carbon pick up of0.09%. 'No reaction occurred in the bath until the excessively hightemperature of 1700 C. was attained, whereupon another 300 pounds ofroll scale was added to the bath: After the re action caused by thisaddition had subsided, a sample was taken which analyzed .07% carbon and8.85% chromium. A slag sample taken at the same time had the followinganalysis: CI' O 60.3% .Fe O 22.9% MnO, 1.0%; SiO .4l%; CaO, nil. Theremainder of the slag was probably MgO. The particular order for whichthis heat wasmade required the following analysis: .08/.12% carbon;.35/.45% manganese; 12/ 14% chromium and .35/.50% silicon, and thesubsequentadditions were ma dc to meet this analysis.

The object of the procedure from this point was to reduce the chromiumback into the metal bath, and this was accomplished by an addition of300 pounds of burnt lime mixed with 150 pounds. of crushed ferro-silicon(containing 50% silicon"). After about twenty minutes the slag formed bythis addition of burnt lime and ferro-silicon appeared light green incolor and very fluid indicating completion of the reduction stage. Anaddition of 20 pounds of ferro-manganese fontaining 80% manganese and.75% car on) was then made to the bath and five minutes after thisaddition was made, the heat was tapped. The final analysis of the metalwas as follows: 09% carbon; 40% manganese; 31% silicon; and 13.5%chromium. The analysis of the final slag was as follows: 01 30 F6203,M110 Slog, 32.8%; A1 0 7.4%; CaO, 52.8% and MgO, 4.0%. The duration ofthe heat from current on until tap was three hours and ten minutes.

(about 1650 C.) iron ore, preferably of low silica content, or cleanroll scale is charged into the furnace in relatively large amount. Theamount of iron ore or scale used depends primarily upon the finalchromium content of the stainless iron and is roughly proportional tothe latter. In the manufacture of a product analyzing 16.5 to 18%chromium I have employed between 1 400 and1' 50 lbs. of roll scale perton (2240 lbs.) of steel scrap charged'or 700 to 875 lbs. per ton ofstainless iron produced. The roll scale readily melts to form a, fluidslag, the first small addition or so of the scale being sutficient todrop the carbon content of the molten steel to a very low figure(approx. ODE-0.04% carbon) in case the scrap initially containedappreciable boil in the open-hearth furnace and is due to" the reactionbetween the carbon originally contained in the ferro-chrome and the ironoxide in the bath. In making stainless iron of 17% chromiumand 0.10%carbon content from high-carbon ferro-chrome analyzing 66% chromium and5.9% carbon, the total ferro-chrome charged amounts to approximately1180 lbs. per ton of steelscrap charged or to 590 lbs. per ton ofstainless iron produced- This quantity of higlrcarbon ferrochrome has,as a rule, been added'intermittently in eight equal additions. Each ofthe eight additions may itself be gradually charged into the furnace,and at times this becomes necessary to avoid a too violent boilingaction. The electrodes are raised when ferro-chrome is added and duringa violent boil in order to avoid undue electrode con: sumption andpossible contamination of the bath with carbon from, the electrodes.

The metalbath should be maintained at a sufliciently high temperatureand the slag sufficiently high in iron oxide to cause the carbon contentof the metal bath to remain always at a comparatively low value. llprefer to operate under such conditions that a metal sample taken fromthe bath ten minutes after a ferro-chrome addition does not show byanalysis more than 0.100.25% carbon. Toward the completion of thisprocedure of charging high-carbon ferro-chrome the rate of carbonoxidation will be retarded appreciably if there is any deficiency in theamount of roll scale or ore originally charged. In such an event,supplementary additions of roll scale may be made in order to bring thecarbon down to the desired percentage. l/Vhen the last addition offerro-chrome has been made, the carbon content is brought down, by anore addition if necessary, to the percentage desired in the finishedproduct, and preferably Mil-0.02% lower than the maximum specified.Occasionally, the carbon content will apparently drop 0.01% or so duringthe second or reducing step, which will be later described, but thiscannot be relied upon and, when it occasionally happens, may be due inpart to unavoidable error in sampling and analysis.

The oxidizing slag at the conclusion of the first step will generallycontain 5 to 20% SiO2, 0 to 15% CaO, 3 to 15% A1 0 1 to 3% M110, 0 to 5%Mg(), and from 45 to 80% of the oxides of iron and chromium combined.There is a consistent increase in the total amount of chromium oxide inthe slag throughout this period of operation, but the actual percentagedepends largely on the manner in which the additions of ore or scalearemade. 7

Analyses of samples taken from the metal bath at various times duringthe oxidizing period of representative heats, together with the chromiumoxide and iron oxide content of several slag samples taken at theparticular times 1n question are givenbelow:

The tabulated data above indicate clearly What importance attaches toeach 0.01% of carbon within the stainless range. It is readily possibleto manufacture by my oxidizing step alone an alloy analyzing 17 chromiumand QTY-0.18% carbon and With a loss of added chromium amounting to onlyabout 30%. In going to an alloy of 0.10% carbon conready described inconnection with the re-- melting of stainless iron scrap because theconditions obtaining in the bath and slag at the end of the oxidationperiod are substantially the same'no matter whether arrived at throughremelting of stainless iron scrap or the addition of high carbonferro-chromc to chromium free scrap.

By the process of the present invention I am able'to produce stainlessiron within the usual commercial analysis limits and with low chromiumlosses by oxidizing the excess carbon and part of the chromium andsubseuently reducing the chromium oxide thus ormed to return thechromium to the bath. Although capable of other applications, theprocess is of particular importance when applied to the remelting ofstainless iron scrap. The production of large quantities of sore isunavoidable in the manufacture of stain ess iron and up to the time ofthe present invention there has been no satisfactory method gojorutilizing it. As a result large tonnages of this very valuable materialhave accumulated and present a serious problem to the stainless ironmaker. By means of my process this Waste material can now be made upinto a marketable product and a saving of considerable magnitudeefiected.

I claim: a

1. The process of making stainless iron which comprises maintaining abath of ferrous metal containing carbon and chromium under anoxidizingslag and at a temperature of superheat, thereby oxidizing asubstantial part of the carbon and chromium, the chromium oxide thusformed passing into the slag, and then, without removing the slag,reducing the chromium'oxide to return a substantial amount of thechromium to the bath.

2. The process of making stainless iron which comprises addingsufficient iron oxide to a superheated bath of ferrous metal containingcarbon and chromium to make it highly oxidizing, thereby oxidizing asubstantial part of both the carbon and chromium, the chromium oxidepassing into the slag, and later, without removing the slag, reducingthe chromium oxide in the slag, thus returning a substantial amount ofthe chromium to the bath.

3. The process of making stainless iron which comprises addingsuflicient iron oxide to a superheated bath of ferrous metal containingcarbon and chromium to make it highly oxidizin thereby oxidizing asubstantial part of both the carbon and chromi- .um, the chromium oxidepassing into the slag, and then adding ferro-silicon and lime to theslag to reduce the chromium oxide and return the chromium to the bath.

4. The process of making stainless iron which comprises forming anoxidizing slag on a superheated bath of iron containing chromium andcarbon in a furnace linedwith magnesia, thereby oxidizing part of thechromium, which passes into the slag as chromium oxide, and the carbonto the extent of holding it at or below the permissible maximum, andthen adding ferro-silicon to the slag to reduce the chromium oxide inthe slag and return a substantial amount of the chromium to the bath. I

5. The process of making stainless iron which comprises forming anoxidizing slag on a superheated bath of iron containing chromium andmore than the permissible carbon, thereby lowering the carbon content toor below the permissible maximum and oxidizing a substantial part of thechromium, which passes into the slag, and then, without removing theslag, reducing a substantial amount of the chromium oxide thus formed toreturn the chromium to the bath.

6. The process of making stainless iron containing not more than 0.12%carbon and not less than 12% chromium which comprises superheating aniron bath containing higher carbon and substantially the requiredchromium, adding sufficient iron oxide to lower the carbon content to orbelow 0.12%, thereby oxidizing a substantial quantity of the chromiumwhich passes into the slag and then adding ferro-silicon to reduce thechromium oxide and return the chromium to the bath, and adding lime tocontrol the acidity of the slag;

7. The process of making stainless iron from stainless iron scrap whichcomprises melting down the scrap, superheating the bath under stronglyoxidizing conditions whereby the carbon content of the bath which hasincreased during melting down is brought back to or below thepermissible maximum, and a substantial part of the chromium oxidized andpassed into the slag, and them without removing the slag, reducing thechromium oxide to return a substantial amount of the chromium to thebath.

8. The process of making stainless iron from stainless iron scrap whichcomprises melting down the scrap, superheating the bath under an ironoxide slag whereby the carbon content of the bath which has in-' creasedduring melting down is decreased to or below the original amount, and asub stantial part of the chromium oxidized, the chromium oxide thusformed assing into the slag, and then adding a re ucing agent to theslag to reduce the chromium oxide and returna substantial amount of thechromium to the bath.

9. The process of making stainless iron from stainless-iron scrap whichcomprises melting down the scrap, adding 1ron made and superheating thebath about 50 to 250 melting down the scrap in the presence of ironoxide, adding additional iron oxide, superheating the bath to about 1700C. whereby the carbon which has been picked up during melting down isoxidized, and the carbon content of the bath lowered to or below theoriginal percentage, and part of the chromium'oxidized and passed intothe slag, and (then adding ferro-silicon to reduce the chromium oxideand return the chromium to the bath, and adding lime to control theacidity of the slag.

11. The process of remelting low carbon, high chromium iron scrap in anelectric furnace without a substantial increase in the carbon ordecrease in the chromium content which comprises melting down the scrapunder superheated and strongly oxidizing conditions, whereby any carbonpicked up during the remelt is oxidized and the carbon content loweredto or below the original figure, while a substantial part of thechromium is oxidized and passed into the slag, and then reducin thechromium oxide thus formed, where y substantially all of the chromium inthe slag is returned to the bath.

In testimony whereof I aflix my signature WALTER M. FARNSWORTH.

